The present invention relates to an imaging device for forming an image on an image sensor such as a CCD sensor or a CMOS sensor. In particular, the present invention relates to an imaging device suitable for uses that obtain an image of an imaging surface tilted relative to an optical axis or an object contacting with the imaging surface.
In these years, public safety concerns and demands for higher security have been increased. Therefore, security technology to authenticate identities has attracted attentions. Among them, there is a long history of studies in fingerprint matching technologies to authenticate identities by comparing individuals' fingerprints obtained through imaging devices for fingerprint authentication with the fingerprints obtained in advance, and such technologies have been practically used. Since the fingerprint matching authentication has relatively high verification precision, it has been first introduced in fields, which require strictly-controlled confidentiality, such as in research organizations, banks, or large companies. Thereafter, with reduction of a price of the fingerprint matching related devices, the fingerprint authentication has been introduced also in smaller companies and general households for higher security upon logging in computers and networks, simplification of operation upon logging in, etc.
As a conventional imaging device for reading fingerprints, for example, there has been known an imaging device disclosed in Patent Reference 1. The conventional imaging device disclosed in Patent Reference 1 is configured being equipped with a light source, a right-angle prism, and a camera.
Patent Reference 1: Japanese Patent Application Publication No. 55-13446
When the conventional imaging device is used, a finger is put on a surface of the right-angle prism so as to have a bundle of rays from the light source enter the right-angle prism at a critical angle. When the bundle of rays enters the right-angle prism, a certain bundle of rays causes total reflection and another bundle of rays does not cause total reflection because of unevenness of a fingerprint. Then, a camera is provided for reading a pattern of the certain bundle of the rays that causes total reflection and the another bundle of the rays that does not cause total reflection, so that it is possible to obtain fingerprint information.
However, in the conventional imaging device for obtaining the fingerprint information using the right-angle prism, the prism surface to put a finger (imaging surface) is tilted relative to an optical axis of a camera lens. Accordingly, trapezoidal distortion or distortion may be generated. When the trapezoidal distortion or distortion is generated, it may cause lower matching precision.
To this end, Patent Reference 2 has disclosed another conventional imaging device as shown in FIG. 12. FIG. 12 is a schematic sectional view of a conventional imaging device. In the conventional imaging device disclosed in Patent Reference 2, a bilateral telecentric optical system 102 is provided between a right-angle prism 100 and an image sensor 101 for minimizing generation of the trapezoidal distortion or the distortion.
Patent Reference 2: Japanese Patent Application Publication No. 08-334691
According to the imaging device described in Patent Reference 2, it is possible to suitably control the trapezoidal distortion or the distortion caused by the right-angle prism. However, it is necessary to provide separately the telecentric optical system composed of a plurality of lenses, thereby making it difficult to reduce a size and a cost of the conventional imaging device.
Moreover, in these days, there has been also available a conventional fingerprint matching device that verifies the identity using fingerprints of four fingers from a pointing finger to a little finger so as to further enhance security. According to the fingerprint matching device, an imaging area to put a finger becomes larger, so that a prism and a telecentric optical system that compose the conventional imaging device unavoidably have to be large. Accordingly, it is necessary to increase a size and a manufacturing cost of the conventional imaging device.
As described above, when the conventional imaging device is used to capture an image of the imaging surface tilted relative to an optical axis or an object contacting with the imaging surface, the trapezoidal distortion or the distortion tends to occur, thereby causing the problems. For example, in order to monitor or capturing an image of a fabrication state of a workpiece placed on a table of a machine tool, it is prefer to provide the conventional imaging device such that the imaging device avoids a cutting tool such as a drill or an end mill disposed above the workpiece. However, the conventional imaging device captures an image of the workpiece in an oblique direction, thereby causing the trapezoidal distortion or the distortion.
In view of the above-described problems of the conventional imaging devices, an object of the present invention is to provide an imaging device having a small size. Accordingly, it is possible to capture an image with suitably corrected aberration even in a case that an imaging surface is tilted relative to an optical axis or an object contacts with such imaging surface.